Rotten fish

Smelly fish seems to be more useful than you might think

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A waft of rotten fish might help doctors looking for signs of awareness in people who are unresponsive after serious brain injuries.

It can be hard to know whether people in such conditions are in a vegetative state, in which they can’t see or feel anything, or whether they have some awareness but are almost paralysed.

Doctors investigate this with tests, such as asking people to follow a moving finger with their eyes. Yet those in a “minimally conscious state”, in which awareness typically fluctuates over time, may be wrongly assumed to be vegetative.


Anat Arzi at the Weizmann Institute of Science in Rehovot, Israel, wondered whether people with some awareness would show a response to smells, as non-injured people reduce the air they inhale in the presence of bad smells, both consciously and unconsciously. We even show a small “sniff response” to odours in our sleep.

“The brain has the capacity to process information when we are not consciously aware, probably for survival reasons,” says Arzi.

Her team investigated 43 people in a rehabilitation facility in the first few weeks or months after a brain injury. About half of them were thought to be in a vegetative state, and the rest were thought to be minimally conscious. A small tube entering the nose could measure the volume of air they inhaled.

Some people in both groups showed about reduction in inhaled air of about 10 per cent when presented with a strong odour, whether it was of liquid that smelled like rotten fish or simply a fruity shampoo.

The test was most revealing for the 24 people classed as vegetative, 16 of whom were later diagnosed as minimally conscious because they had started showing some responses, such as eye movements. Ten of these people had showed a sniff response, but none of those who stayed in a vegetative state had demonstrated one.

“If you don’t have a sniff response, we can say nothing. But if you have a sniff response, it’s highly informative,” says Arzi. “Maybe we can provide with the sniff test a simple tell to be used at the bedside that will reduce misdiagnosis rates.”

Journal reference: Nature, DOI: 10.1038/s41586-020-2245-5

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